It is well known in a variety of industries to use heaters having burner assemblies for a number of different applications, including kilns, drying apparatus, furnaces and for preventing freezing of tanks and pipelines. In the oil and gas industry, heaters are particularly used in regions where low ambient temperatures may result in freezing of storage tanks or in production and process pipelines. Further process heaters are used which may be used when knocking water out of oil and when heating salt baths and the like. Gas burner assemblies are typically arranged in a housing or firetube which extends into a storage or holding tank to be heated.
In prior art natural draft or “non-forced draft” situations, primary combustion air is drawn into a mixing chamber or mixer head of the gas burner assembly as a result of the velocity of the flammable gas entering the mixing chamber or venturi. The premixed gas/air fuel mixture exits the venturi at a burner nozzle, typically a rosebud nozzle, where the mixture is ignited. Secondary combustion air is drawn into the housing and around the burner assembly as a result of draft. The secondary air, intended to aid in combustion, may adversely affect the operation of the burner assembly. Large volumes of secondary air creating a large turbulent draft at the burner head may result in the flame being lifted from the burner nozzle or may blow out a flame at the nozzle. Attempts to reduce or dampen the amount of secondary air entering the burner can substantially shutoff the flow of secondary air which compromises the efficiency of the burner.
Further, variability in operation can adversely affect the consistency of ignition and flame sensing. Typically, burners may be operated in high-fire and low-fire situations. In a low-fire situation, the pressure of fuel entering the burner is relatively low compared to a high-fire situation. Conventional burners which are set to operate under low-fire conditions can experience lifting of the flame from the burner nozzle should they be used in a high-fire situation. Thus, in conventional burners, ignition and flame sensing, which are optimized for one flame characteristic, become problematic as the position of the flame alters. Use of a pilot has provided a consistent flame source and ignition sensing. In variable firing conditions, should the fuel/air ratio and secondary air flow be sufficiently unstable at the burner nozzle, remote lighting of the burner becomes difficult. As a result, the industry has typically relied on manual lighting of such burners which has resulted in significant hazard to personnel performing the task.
Additionally, freezing is a common problem with natural draft burner assemblies. Typically, areas of low pressure adjacent the orifice of the burner may result in freezing at the orifice or in the gas lines which feed the orifice. Low flow of fuel at pilot assemblies are even more prone to freezing
Clearly, there is interest in the industry to provide a reliable burner which remains lit under ambient conditions, is safe to ignite and operate and permits flame-sensing in both low fire and high fire situations, does not freeze in low ambient temperature and is efficient.